Known injection devices are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Generally, such injection devices require a high force drive spring in order to reliably empty the syringe in the time before retraction of the syringe. When the drive spring is first released, the spring first takes up clearance in the syringe, then extends the syringe and needle and then delivers the drug. The spring force is significantly higher than that required for these initial steps and excess energy is liberated in the form of noise and vibration resulting from recoil of the spring.
An injection device can generally operate with a range of syringe fill volumes. When the fill volume is low, there can be substantial clearance to be taken up and hence louder noise and higher recoil on actuation.
It is therefore desirable to minimise the noise and recoil to avoid startling the patient for a range of syringe fill volumes.